Paper presented at the Second Gulf Conference On Roads, Abu Dhabi, March 2004,

Transcription

1 Paper presented at the Second Gulf Conference On Roads, Abu Dhabi, March 2004,

2 Authors: Khalid Ahmed Meccai Eyad Al Hasan Khalid Meccai obtained his Master s degree in Technology with specialization in Geotextiles in With over 18 years experience in geotextiles, he is currently heading the Marketing and Technical Services department at Alyaf. Eyad Al Hasan obtained his Bachelor's degree in Civil Engineering from U.K. in He is currently working with Alyaf as Sales Manager. Summary: Geotextiles play a significant part in modern pavement design and maintenance techniques. The growth in their use worldwide for transportation applications in particular, has been nothing short of phenomenal. The focus of this paper is on better understanding of this relatively new tool available to the transportation engineer. The paper provides an overview of the current geotextile technologies and highlights the functions geotextiles perform in enhancing the performance and extending the service life of paved roads. Three key application areas of geotextiles, construction of pavements, in asphalt concrete overlays and for drainage systems along with impetus on the current design methodologies available in geotextile design and selection are addressed. Keywords: Geotextile, pavement, drainage, reflective cracking, subgrade, functional properties. 1. Introduction Geotextiles have proven to be among the most versatile and cost-effective ground modification materials. Their use has expanded rapidly into nearly all areas of civil, geotechnical, environmental, coastal, and hydraulic engineering. They form the major component of the field of geosynthetics, the others being geogrids, geomembranes and geocomposites. The ASTM (1994)[1] defines geotextiles as permeable textile materials used in contact with soil, rock, earth or any other geotechnical related material as an integral part of civil engineering project, structure, or system. Based on their structure and the manufacturing technique, geotextiles may be broadly classified into woven and nonwoven. Woven geotextiles are manufactured by the interlacement of warp and weft yarns, which may be of spun, multifilament, fibrillated or of slit film. Nonwoven geotextiles are manufactured through a process of mechanical interlocking or thermal bonding of fibers/filaments. Mechanical interlocking of the fibers/filaments is achieved through a process called needle punching. Needle-punched nonwoven geotextiles are best suited for a wide variety of civil engineering applications and are the most widely used type of geotextile in the world. Interlocking of the fibers/filaments could also be achieved through thermal bonding. Heat-bonded geotextiles should be used with caution, as they are not suitable for filtration applications or road stabilization applications over soft soils [2]. 2. Geotextile Functions: The mode of operation of a geotextile in any application is defined by six discrete functions: separation, filtration, drainage, reinforcement, sealing and protection. Depending on the application

3 the geotextile performs one or more of these functions simultaneously. The protection function is not discussed here as it is not related to transportation applications Separation: Separation is defined as, The introduction of a flexible porous textile placed between dissimilar materials so that the integrity and the functioning of both the materials can remain intact or be improved (Koerner, 1993) [3]. In transportation applications separation refers to the geotextile s role in preventing the intermixing of two adjacent soils. For example, by separating fine subgrade soil from the aggregates of the base course, the geotextile preserves the drainage and the strength characteristics of the aggregate material. The effect of separation is illustrated in figure 1. Figure 1, Concept of separation Function 2.2. Filtration: It is defined as the equilibrium geotextile-to-soil system that allows for adequate liquid flow with limited soil loss across the plane of the geotextile over a service lifetime compatible with the application under consideration (Koerner, 1993) [3]. To perform this function the geotextile needs to satisfy two conflicting requirements: the filter s pore size must be small enough to retain fine soil particles while the geotextile should permit relatively unimpeded flow of water into the drainage media. A common application illustrating the filtration function is the use of a geotextile in a pavement edge drain, as shown in figure 2. Figure 2 Filtration and Transmissivity Functions

4 2.3. Drainage (Transmissivity): This refers to the ability of thick nonwoven geotextile whose three-dimensional structure provides an avenue for flow of water through the plane of the geotextile. Figure 2 also illustrates the Transmissivity function of geotextile. Here the geotextile promotes a lateral flow thereby dissipating the kinetic energy of the capillary rise of ground water Reinforcement: This is the synergistic improvement in the total system strength created by the introduction of a geotextile into a soil and developed primarily through the following three mechanisms: One, lateral restraint through interfacial friction between geotextile and soil/aggregate. Two, forcing the potential bearing surface failure plane to develop at alternate higher shear strength surface. And three, membrane type of support of the wheel loads Sealing Function: A nonwoven geotextile performs this function when impregnated with asphalt or other polymeric mixes rendering it relatively impermeable to both cross-plane and in-plane flow. The classic application of a geotextile as a liquid barrier is paved road rehabilitation, as shown in Figure 3. Here the nonwoven geotextile is placed on the existing pavement surface following the application of an asphalt tack coat. The geotextile absorbs asphalt to become a waterproofing membrane minimizing vertical flow of water into the pavement structure. Figure 3 Sealing Function 3. Design Properties and Tests: Standardized testing of geotextile properties has evolved over a very short time reflecting the increasing rate with which these materials are used. Sufficient number of standardized tests both ASTM [1] and EN [4] are available with which to assess the suitability of the geotextile to the specific application. The design engineer incorporating geotextiles needs to understand these test methods and specify only those properties that govern the functional needs of the end application. By taking these steps, the engineer not only protects the client s interest in getting the right product for the application but also invites the largest number of geotextile manufacturers

5 possible; thereby assuring that the cost of the construction materials is kept to a minimum. In this paper only the functional properties that govern typical geotextile applications are addressed Puncture Strength: ASTM D This test is intended to measure the puncture resistance of geotextiles and geomembranes and simulates the puncture strength of the geotextiles to static loads of aggregates. In this test the geotextile is secured in a ring clamp. A steel rod with a conical tip is then forced through the material and the resistance to puncture is measured in Newton Burst Strength: ASTM D This test simulates the strength of the geotextile to a continuous hydraulic/mechanical load. In this test the geotextile sample is secured over an inflatable membrane. As the membrane is inflated, the geotextile deforms to a hemispherical shape. The force causing rupture of the geotextile is recorded in units of pounds per square inch or kilo Pascal Dynamic Puncture: EN 918. This test is intended to measure the strength of the geotextile to falling objects and simulates the placement of aggregates over the geotextile during the installation stage. In this method the geotextile is fixed in a ring clamp and a steel cone of 1000 grams is dropped from a height of 500 centimeters over the geotextile. The diameter of the hole created by the impact is measured and expressed in millimeters. Note, the smaller the hole size the tougher the geotextile is to dynamic loading Grab Tensile Strength and Elongation: ASTM D This index test measures the tensile strength and elongation along the plane of the geotextile by loading it continually. In this test the specimen sample of dimensions 100 by 200 millimeters is secured in clamps of a tensile testing instrument and loaded with a constraint strain rate of 300 millimeters per second. The value of the breaking load is expressed in Newton and the elongation at break in percent Permeability: This test is intended to measure the rate at which liquids can pass through the geotextile. The test method ASTM D 4491 measures the permittivity, which is related to permeability, by the following equation. = k / t - (1) Where: = geotextile permittivity (sec-1) k = geotextile permeability (cm/sec) t = geotextile thickness (cm)

6 Alternatively the test method BS 6906 Part 3 measures the permeability as flow rate and the value expressed in liters per square meters per second Apparent Opening Size (AOS): ASTM D 4751 The apparent opening size reflects the approximate largest opening dimension available through which the soil may pass. In this test method, the geotextile is secured between two US Standard Sieves. Glass beads are then placed in the upper screen and sieved through the geotextile. The AOS corresponds to the bead size for which no more than 5% pass through the geotextile. The AOS is expressed as US Standard Sieve Number or in millimeters. 4. Areas of Application: This paper addresses the following major application areas of geotextiles in transportation engineering: Flexible paved road construction, Drainage Applications and Pavement overlays. Other transportation related geotextile applications like erosion control of slopes both in waterways and dry areas have not been covered in this paper Flexible Paved Road Construction Geotextiles extend the service life of roads, increase their load-carrying capacity, and reduce rutting. The effectiveness of geotextiles in stabilization and separation roles with flexible pavements has been extensively researched at Virginia Tech. (Smith et al, 1995[5]; Lacina, 1997[6]; Valentine, 1994[7]). The researches found that for weak subgrades (CBR = 2%) the geotextile extends the service life of a flexible pavement section by a factor of 2.5 to 3.0 compared to a non-stabilized section. Further the research found that a geotextile effectively increased the pavement section s total AASHTO structural number by approximately 19 %. Research on the effect of geotextile in pavement sections with moderate strengths (CBR = 4.2 to 4.5 %) found that the geotextile increased the service life of the pavement section by a factor of 2.0 to 3.3 and the AASHTO structural number increased by 13 to 22%. These significant improvements are obtained primarily through the separation function of the geotextile placed at the interphase of the base course aggregate and subgrade soil. Without a separator geotextile, the aggregate layer becomes contaminated with fines from the subgrade. This contamination leads to the development of a new soil-aggregate layer at the interface whose strength is less than that of the aggregate layer. The loss of strength occurs because granular aggregates (gravel, sand, etc.,) obtain their shear strength primarily through the point-to-point contact of adjacent particles. As the volume of fines increases, the shear strength of the aggregate mixture increases because the fines help to distribute shear stress. However as the fine soil content further increases the stress is distributed primarily through the soil fines, which have considerable lower strength. With geotextile acting as a separation / filtration layer at the interphase of the subgrade and the drainage aggregate, the aggregate base course layer is completely insulated from the soil fines, therefore the designed base course properties are maintained throughout the life of the project. The system performance is also improved through the secondary functions of drainage (the geotextile allowing excess pore pressures to dissipate through the Transmissivity function) and the reinforcement function of the geotextile.

7 Pavement Section Design: The design of pavement sections incorporating geotextiles can be performed using many of the current design methodologies. The AASHTO (1986)[8] design method is modified to account for the contribution of geotextile and can be found in the FHWA publication Geosynthetic Design and Construction Guidelines (Holtz et al, 1995)[9]. Figure 4, Base Course Being Laid Over Geotextile Geotextiles also play a major role in construction of paved roads over areas having high ground water table. Drainage of water from pavements has always been an important consideration in road design; however current methods of pavement design have resulted in base courses that do not drain well. The problem has been compounded with the rise in the water table. Water rises up into the base course through pore water pressures and through capillary function leading to saturation of the base courses. Saturation of the base course changes the dynamics of vertical stress distribution, and may allow transfer of traffic loads directly to subgrade soil, eliminating the benefits of the structural layers leading to rapid pavement distress. A pavement layer, which is saturated 10% of its time, will have its service life reduced by as much as 50% (Cedergreen 1974) [10]. Thus eliminating saturation of the base course is a prudent design objective. AASHTO also recognizes the harmful effects of water in the pavement structure. In the AASHTO design method, the effective structural number (SN eff ) of pavement base and subbase materials, which drain well, is increased, whereas if materials drain poorly, the effective structural number is decreased (AASHTO, 1993)[11]. While drainage factor for excellent drainage can be 1.20, Drainage factor for poor drainage can be as low as This means by providing proper drainage the design strength of pavement base and sub-base materials can be doubled. To adequately address the ground water drainage the designer needs to consider providing subsoil drainage systems as illustrated in figure 2 that not only lead the ground water away from the pavement structural layers but also prevent the capillary rise of the ground water into the structural layers. It is recommended that the structural drains be connected to shoulder edge drains that lead the water away. The location of the trench drain can vary depending on actual site conditions. Design of these subsurface drains is detailed subsequently in this paper.

8 Geotextile Requirement: Selecting a geotextile for paved road depends upon the geotextile survivability. If a roadway system is designed correctly, then the stress at the top of the subgrade due to the weight of the aggregate and the traffic load is less than the bearing capacity of the subgrade plus a safety factor. However the stresses applied to the subgrade and the geotextile during the construction is much greater than that applied during service. Therefore, selection of the geotextile in roadway applications is usually governed by the anticipated construction stresses. The geotextile must survive the construction operations if it is to perform the intended function of separation and filtration. The geotextile requirements under moderate and severe conditions are provided in Table 1. These survivability requirements are based on properties of geotextiles, which have performed satisfactorily. Judgment and experience our required to select final specification values. For large projects geotextile survivability should be verified by conducting field tests under sitespecific conditions. The selected geotextile must also retain underlying soil fines, while allowing a relatively unimpeded flow of water. Hence the geotextile Opening size and Permeability should be ascertained to match the site requirements. PROPERTY TEST REQUIREMENTS UNIT METHOD MODERATE HIGH Puncture Strength ASTM D4833 N Dynamic Puncture ENISO (Minimum hole diameter) 918 mm Ultimate Elongation ASTM D4632 % >50 >50 Burst Strength ASTM D3786 kpa Grab Strength ASTM D4632 N Permeability ASTM D 4491 cm/sec > than the soil permeability Apparent Opening Size ASTM D4751 mm < 0.15 Table 1 Geotextile Requirements for Roadway Applications 4.2. Pavement Overlays Asphalt concrete pavement overlays can benefit from the use of paving fabric interlayer. The documented field experience indicates to a number of positive benefits including: Waterproofing of the lower layers, thereby maintaining higher material strengths. Retarding reflection cracking in the overlay by acting as a stress absorbing membrane interlayer. Increase in structural stability by providing for more stable subgrade moisture contents. Paving fabrics can also be used in new pavements to provide the same benefits. If fabric is added and the overlay thickness is not reduced from that determined by normal methods, than an increase in performance can be obtained.

9 Capabilities Of Paving Fabrics: The inclusion of a nonwoven paving fabric interlayer system significantly improves the performance of asphalt concrete overlays. This performance improvement is a result of both the waterproofing capabilities and the stress absorption capabilities of the paving fabric system. (Maxim Technologies Report)[12]. Synthetic fabrics and stress-absorbing interlayer (SAMI) have been effective in controlling low to medium severity alligator cracking. They may be also useful for controlling reflection of temperature cracks when used in combination with crack filling. They generally do little, however, to retard reflection of cracks subject to significant horizontal or vertical movement. AASHTO Guide for Design of Pavement Structures 1993[11]. Figure 5 Paving Fabric Installation Both laboratory and field pavement cores indicate that the presence of a properly installed paving fabric interlayer reduces the permeability of a pavement by one to three orders of magnitude. By reducing the infiltration of moisture the paving fabric maintains the strength of the subgrade, subbase and base course, limiting damage due to saturated condition pore pressures (Mark and Thomas, TRB circular 1999) [13] Properties of Paving Fabric: Paving fabrics are nonwoven fabrics from grades ranging from 135 gm/m2 to 200 gm/m2. The lighter fabrics when impregnated with asphalt primarily function as moisture barrier. Use of heavier, nonwoven geotextiles provides cushioning or stress-relieving membrane benefits in addition to moisture-barrier functions. Minimum properties required for paving fabrics as per AASHTO M , Standard specifications for geotextiles [14] are provided in Table Installation procedure: The surface on which the paving fabric is to be placed should be free from dirt, water, vegetation or other debris. Cracks are filled or repaired and tack coat typically ranging from 1 to 1.35 l/m2 of residual asphalt is applied evenly on the surface. The paving fabric is then laid on

10 the surface with minimum wrinkles, and placement of the hot mix overlay shall closely follow the fabric lay down. PROPERTY TEST METHOD UNITS REQUIREMENTS Grab Strength ASTM D4632 N 450 Mass per unit area ASTM D3776 gm/m Ultimate Elongation ASTM D4632 % >50 Asphalt Retention Texas DOT 3099 l/m Melting Point Table 2 Paving Fabric Requirements 4.3. Drainage: ASTM D276 0 C 150 The removal of water is important to the success of many civil engineering problems. In transportation applications, if the base course does not drain rapidly enough, stress from the traffic loadings is transferred to the subgrade with little or no reduction, resulting in accelerated road failure. The removal of water must be performed in a controlled fashion. Otherwise, severe erosion, piping, or settlement of soils may result in undermining adjacent structures. To accomplish this task the drainage system should fulfill two criteria: Have maintained permeability by providing relatively unimpeded flow of water. Filtration of base soil by preventing the migration of soil fines into the drain. These criteria can be met by using several layers of specially graded aggregates. This often proves to be extremely expensive requirement to meet. The same result can be achieved at a fraction of the cost by using selected geotextiles, which act as filters around the aggregate drainage system. The introduction of geotextile lined drainage systems has enhanced the technical benefits and economical application of blanket and trench drains under and adjacent to pavement structures. The excellent filtration and separation characteristics associated with geotextiles permits the use of a single layer of open graded aggregate base or trench aggregate enveloped in a geotextile. Figure 6 Pavement Edge Drains

11 4.4. Geotextile Filter Design: Designing with geotextiles for filtration is essentially the same as designing with graded filters. Based on the analogy to soil filter design criteria, the following design criteria for geotextiles is stated: The geotextile must retain the soil fines (retention criterion), while Allowing a relatively unimpeded flow of water (permeability criterion), throughout the life of the structure (clogging resistance criterion). To perform effectively, the geotextile must survive the installation process (survivability criterion). The design procedure proposed is based on the above stated parameters and the design procedure developed by Christopher and Holtz (1985) [9] Retention Criteria Under Steady State Flow The retention criterion is governed by the Apparent Opening Size (AOS) of the geotextile. AOS or O 95 (geotextile) < B D 85 (soil) - (2) Where: AOS or O 95 = apparent opening size (mm); O 95 = opening size in the geotextile for which 95% are smaller (mm); B = a coefficient (dimensionless) ranging from 0.5 to 2 depending on the type of soil; and D 85 = soil particle size for which 85% are smaller (mm) Clogging Resistance: The criterion for clogging resistance is: Geotextile porosity, n > 70%. For severe / critical conditions where soils potential to clog are addressed it is recommended that soil-geotextile clogging tests like the gradient ratio test [ASTM D 5102] [1] are conducted Permeability Criteria: Permeability requirements: For less critical applications: k geotextile > k soil - (3) For critical applications: k geotextile > 10 k soil - (4) Where: k soil is the Darcy s coefficient of permeability (m / sec.) of the soil to be filtered. k geotextile is the permeability coefficient of the geotextile.

12 Survivability Criteria: Survivability refers to the geotextile s ability to withstand the installation stresses and perform as intended in the project design. Table 3 gives the minimum physical property requirements for drainage applications. These minimum survivability requirements are based on the experience on the properties of geotextiles, which have known to perform satisfactorily in these applications. Property Test method Unit High Survivability Moderate Survivability Puncture strength ASTM D4833 N Dynamic puncture (hole diameter) EN 918 mm < 14 < 18 < 22 Burst strength ASTM D3786 psi Elongation at break ASTM D4632 % Notes: 1. High-survivability drainage applications for geotextiles are where installation stresses are more severe than moderate applications, i.e., very coarse, sharp angular aggregate is used, a heavy degree of compaction (>95%) is specified, or depth of trench is greater than 3m. 2. Moderate-survivability drainage applications are those where geotextiles are used with smoothgraded surfaces having no sharp, angular projections, no sharp angular aggregates is used, compaction requirements are light, (less than 95%), and trenches are less than 3m in depth. Table 3 Physical Requirements for Drainage Applications 5. Conclusion: Geotextiles are effective tools in the hands of the civil engineer that have proved to solve a myriad of geotechnical problems. With the availability of variety of products with differing characteristics, the design engineer needs to be aware of not only the application possibilities but also more specifically the reason why he is using the geotextile and the governing geotextile functional properties to satisfy these functions. Design and selection of geotextiles based on sound engineering principles will serve the long-term interest of both the user and the industry. 6. References: [1] ASTM (1994), Annual Books of ASTM Standards, American Society Testing and Materials, Philadelphia, Pennsylvania. Volume 4.08 (1), Soil and Rock. Volume 4.08 (2), Soil and Rock, Geosynthetics, Volume 7.01, Textiles. [2] GREGORY RICHARDSON N., BARRY CHRISTOPHER R., Geotextiles in Transportation Applications, Featured Short Course, [3] KOERNER, R. M., Designing with Geosynthetics, Third edition, Prentice Hall, 1993.

13 [4] EUROPEAN COMMITTEE FOR STANDARDIZATION (CEN), Geotextiles and geotextile related products, central Secretariat: rue de Stassart 36, B-1050, Brussels. [5] SMITH T.E., BRANDON T.L., AL-QADI I.L., LACINA B.A., BHUTTA S.A., HOFFMAN S.E., 1995, "Laboratory Behavior of Geogrid and Geotextile Reinforced Flexible Pavements; Final Report," Virginia Tech, Blacksburg, VA. [6] AL-QADI I.L., BRANDON T.L., BHUTTA S.A., LACINA B.A. [1997]. "Geosyntically Stabilized Flexible Pavements": Virginia Tech Study, Proceedings, Geosynthetics 1997 Conference, Long Beach. [7] AL-QADI I.L., BRANDON T.L., VALENTINE R.J., LACINA B.A., SMITH T.E., [1997]."How Do Geosynthetics Improve Pavement Performance": Proceedings of Infrastructure: New Materials and Methods for Repair, ASCE, San Deio, pp [8] AASHTO Guide for the Design of Pavement Structures, American Association of State Highway and Transport Officials, Washington DC, [9] ROBERT HOLTZ D., BARRY CHRISTOPHER R., RYAN BERG R., Geosynthetic Engineering, Bitech Publishers Ltd. Canada, 1997 [10] CEDERGREEN H. R., Drainage of Highway and Airfield Pavements, John Wiley and Sons., New York, NY, [11] AASHTO Guide for the Design of Pavement Structures, American Association of State Highway and Transport Officials, Washington DC, [12] MAXIM TECHNOLOGIES INC, Nonwoven Paving Fabrics Study Final R e p o r t, December <WWW@ gmanow.com/pages/desconcept.asp> [13] MARK MARIENFIELD L., THOMAS BAKER L., Paving Fabrics Interlayer System as a Pavement Moisture Barrier, Transportation Research Board, [14] AASHTO (1996) Standard Specifications for Geotextiles - M Standard Specifications for Transportation Materials and Methods of Sampling and Testing, American Association of State Transportation and Highway Officials, Washington D.C., pp